Portraits are tricky things. Even when the quality is impressive, they often do not fetch as much as you would expect. After all, who wants a portrait of someone else's relative? The exceptions to this rule are where the subject is particularly attractive, or the artist is exceptionally well-regarded, for example, Vincent Van Gogh, or John Singer Sargent. The portrait you are enquiring about may be beautiful, but it's impossible to assess its value without seeing it n person. I recommend you show it to an art sales professional in your local area who may be able to give you further advice.